Power-and-free conveyor systems conventionally include a plurality of carriers each consisting of one or more trolleys carried for free motion along a free conveyor track. A power conveyor is disposed either above the free conveyor track in an overhead system, or beneath the free conveyor track in an inverted system, for selective engagement with trolleys on the free conveyor track. In this way, the trolleys are propelled along the free conveyor track by engagement with the overlying or underlying power conveyor. A problem is encountered when the power conveyor becomes disengaged from the free conveyor trolleys, particularly on declined portions of the conveyor system. Under such conditions, gravitational forces on the trolley and the load carried thereby can increase trolley velocity to an uncontrolled runaway level. Various mechanisms have heretofore been proposed for preventing or arresting such a runaway condition. For example, it is conventional practice to position the power conveyor track closer to the free conveyor track in inclined portions of the conveyor system so that the pusher on the power conveyor will nest more deeply between the associated dogs on the free conveyor trolley. However, the power chain can break, or a chain surge or other malfunction can cause the trolley dogs to become disengaged from the power conveyor.
It has been proposed to provide a safety conveyor mechanism adjacent to the tree conveyor on declined portions of the conveyor system. The safety conveyor has retarder dogs that are positioned to be engaged in turn by the trolleys on the free conveyor track, and to prevent the trolley from running away in the event of power conveyor malfunction. Such a safety conveyor adjacent to each descent of a conveyor system greatly increases overall system cost and complexity. Another device for preventing trolley runaway heretofore proposed comprises a series of dogs pivotally disposed on the tree conveyor track in the area of each decline. The trolley normally engages each dog in turn as it passes, and each dog pivots upwardly out of the way and then swings back into position. However, if the trolley is traveling at an excessive speed, the dog is flipped completely over by the momentum of the trolley, and engages the trolley to block further movement. These dogs are less expensive than separate safety conveyors, but tend to jerk the trolley to a stop, potentially damaging the trolley, the system and the trolley load. It has also been proposed to provide a bar on the free conveyor track that is releasably movable laterally into engagement with the wheels or rollers that guide the trolley along the track. Such devices cause excessive wear to the trolley guide wheels, and do not take synergistic advantage of trolley momentum to arrest trolley motion.
It is a general object of the present invention to provide a method and apparatus for arresting a runaway trolley condition in a power-and-free conveyor system, particularly in declined portions of such a system, that are adapted to cooperate synergistically with momentum of a runaway trolley automatically to increase forces arresting the runaway trolley motion. A more specific object of the present invention is to provide a method and apparatus of the described character for arresting a trolley runaway condition in a power-and-free conveyor system that are economical to implement, that are readily adapted for both overhead and inverted power-and-free conveyor systems, that do not engage the trolley guide wheels or other moving parts of the trolley and thus reduce trolley wear, that gradually arrests trolley motion and thus do not cause undesirable jerking of the trolley and trolley load, and that are fail-safe in operation in that motion of trolleys along the conveyor decline will automatically be arrested in the event of system power failure.